1. Field of the Invention
The invention relates generally to test probe heads and more particularly to high density probe heads using deformable technology with isolated signal lines.
2. Description of the Prior Art
Test probe heads are used in the semiconductor industry to transmit electrical test signals from a test system to and from electrical circuits on miniature electronic devices. By this process the circuitry of the electronic device can be tested during and after the manufacturing process and defective devices identified. The test probe heads are also used to test completed devices after receipt and may be used by a purchaser to check devices prior to inserting them into other assemblies.
Typically, the devices under test are rectangular dies on which a circuit has been implanted. Associated with such a die are a plurality of pads (usually between two and five mils square) which provide a means for electrical access to the circuit on the die. Test probe heads are generally mounted on a machine commonly referred to as a wafer probing machine. The testing operations include positioning a wafer or other electronic device to be tested on the chuck of the machine. With the machine in operation, the tip contacts of the probes are guided relative to the pads of the device under test to make the interface engagement of the tip contacts with the pads. Upon making interface contact, the electronic test equipment electrically coupled through the probe-head assembly evaluates the electrical operation of the device. When testing is complete the tested device is moved on relative to the probe-head assembly and another device is placed in position to repeat the testing operation.
In one type of wafer probing machine currently in use, the chuck is movable in a Z-axis by a fixed amount only, i.e. when actuated the chuck will move upward in the Z-axis a fixed distance, typically ten to twenty mils. The operator positions the probe-head above the wafer on the chuck at a predetermined distance. This distance is selected such that the fixed travel of the chuck will cause the pads of the device under test to just come into contact with the probe-points on the test head and then travel an additional distance to make proper electrical contact between the probe points and the pads. This additional movement is known as "overdrive". The purpose for "overdriving" the pads on the device under test beyond the point of initial contact is to ensure proper electrical contact between the probe tips and pads by making up for variations in wafer thickness over the surface of the wafer and differences in the coplanarity of the probe tips. The fixed amount of travel is set by the manufacturer of the machine while the "overdrive" is set by the user. The unadjustable nature of the travel of the chuck can create a problem. For example, if the operator initially places the probe-head too close to the chuck, the "overdrive" can damage the probe-points or the pad surfaces resulting in the inability to transmit test signals. Additionally, the rapid up and down motion of the chuck necessary for the efficient testing of a plurality of miniature electronic devices on the wafer creates vibrations which can cause the probe-points to oscillate up and down on the pads and thereby disrupt the testing process.
In another type of wafer probing machine currently in use, the distance of chuck travel in a Z-axis is automatically controllable. The machine senses when the probe-points are just touching the pads by means of an edge sensor and then the chuck's movement is continued upward a preprogrammed "overdrive" distance. Thus, the overdrive distance is the same every time even if the operator initially incorrectly sets a space between the probe-head and the device under test. However, the vibration problem discussed above continues to exist and provision must be made for vertical movement of the probe-points in response to the overdrive.
In general, any process which utilizes overdrive requires that the probe-points and/or probe arms be deflectable in the Z-axis to partially compensate for the overdrive motion and to prevent damage to the points and/or the pads, whether it is the movement of the wafer or the movement of the probe-head which causes the overdrive.
In the past test probe-heads have used thin wires as probes with the tip of the wire contacting the pads of the device under test. These wire tips act as springs to handle movement at least in the Z-axis and to maintain the necessary pressure on the pad for proper electrical connection. Several problems with such wire probes became evident. The wires were easily bent out of alignment. As the density of pads is increased it becomes necessary to reduce the wire size so that a greater number of wire probes can fit within the same area as before. However, the reduction in wire size results in the loss of mechanical strength such that the use of wire probes has become impractical in a high density situation when the wire size is very small (below six mils).
In U.S. Pat. No. 3,832,632 issued to the present applicant, a deformable probe-point which compensates for overdrive is disclosed. The probe arm is fixed with the probe-point being a hemispherical structure composed of an elastic compressable material. Particles of rhodium were suspended in the elastic conductive material to provide sharp edges to penetrate the oxide layer on a pad in order to ensure proper electrical contact and to provide a very large area of contact for use of high current when the point was compressed against the pad. While this structure was capable of providing electrical connection between a test device and a device under test, the probe-points were fairly fragile with respect to the adhesion of the elastic conductive material to the probe arms. The points were easily displaced thereby destroying the electrical contact between the signal line and pad.
Another approach to providing a probe which is deflectable in the Z-axis is by use of a probe mounted on a deflectable member. Such a device is disclosed in U.S. Pat. No. 3,405,361, issued to Lionel K. Kattner, et al. Kattner uses a fluid under pressure to expand a deflectable member downward bringing probes mounted thereon into contact with the device under test. A problem with this approach is that the fluid activation system tends to tear the flexible material comprising the deflectable member away from the rigid support member to which it is attached resulting in failure of the deflectable member. Another problem is that contact points are required to be plated onto the probes resulting in a hemispherical shape. Such hemispherical probe-points result in poor electrical contact. This poor electrical contact occurs because the surface of the hemispherical tips making contact with the device under test become flattened as a result of normal wear. This increases the area of the contact surface which means that for a given force, the pressure over the contact surface is decreased. As such pressure is decreased, contact resistance increases causing difficulties in getting test signals to and from the device under test. Another problem with such hemispherical probe-points is that they cannot be produced small enough or in the proper proportions to fit on a typical pad resulting in poor electrical contact therewith. Current fabrication techniques require the use of glass passivation which limits the target area for point contact on the pad.
Existing probes are unshielded making them susceptible to sources of electrical noise. Furthermore, the probe arms are not designed to match the impedance of the test system. As the frequency of the test signal increases (typically above five megahertz), the test signal tends to deteriorate. Such deterioration may be caused by external random electrical noise, cross talk between adjacent probe arms or by an impedance mismatch in the test line connecting the test device and the device under test. Even where impedance is matched, signal reflections can interfere with testing if the signal line is not properly terminated.